Power generating or other plants circulate fluids, such as water or steam, through tubes or banks of tubes. The fluid is often heated and used, for example, to drive turbines which generate electricity.
The tubes are generally arranged in large panels or banks of parallel tubes which are connected together with a metal membrane or web continuously interposed between each pair of adjacent tubes in the bank to form a tube wall. The tubes generally have an outer diameter which can range from about 1 inch up to about 3 inches, with a wall thickness which can be up to about 0.5 inch. The web or membrane connecting adjacent tubes to each other generally has a thickness about equal to the wall thickness of the tubes, with the width of the webbing generally ranging from about 0.25 inch to about 0.75 inch. The webs or membranes are generally welded to the outer walls of adjacent tubes to form the tube banks. In some cases a weld overlay comprising a metal or alloy is present on portions of the tube bank in an effort to prevent corrosion or deterioration. The weld overlay is present on both sides of a tube bank in some embodiments. On account of deterioration due to corrosion and the like, the tubes occasionally require replacement. Ordinarily, repair of damaged or worn tubing involves cutting and removal of relatively large sections of the tube banks, and replacement with a new panel of tubes. The bank sections are generally replaced in large rectangular sections typically having sizes ranging from a few feet to 10, 20, 100 or even 200 feet in length.
The sections of tube banks which are to be replaced are generally cut out using a power saw or a cutting torch. A method and apparatus for cutting out a section of boiler tube wall is illustrated and described in U.S. Pat. No. 5,033,347, herein fully incorporated by reference.
After the damaged or worn section of tube wall which is to be replaced has been cut out and removed, it is necessary to properly prepare the tube ends exposed by removal of the old section before positioning the new tube bank section in the tube wall opening. Proper preparation of the exposed tube ends of the existing boiler tube wall requires chamfering or beveling of the exposed tube ends to facilitate a good weld between the new tube wall section and the existing boiler tube wall. More specifically, the adjoining tube ends of the existing tube wall and those of the new or replacement section of tube wall should be provided with a frustoconical bevel so that when the adjoining tube ends are placed in alignment, a circumferential groove is formed to receive molten metal from the welding rod. The replacement section of tube wall is usually fabricated and prepared for welding off-site before the damaged or worn section of existing tube wall is removed. The exposed tube ends of the existing tube wall, however, must be prepared for welding on-site, often in a confined space. It is highly desirable that all work performed on the existing tube wall be conducted as quickly and efficiently as possible so as to minimize the period during which the steam generator is taken out of service for repair. Accordingly, preparation of the exposed tube ends of the existing tube wall for welding is generally accomplished using portable hand-held milling tools such as those disclosed in U.S. Pat. Nos. 4,449,871 and 4,889,454, herein fully incorporated by reference.
To facilitate rapid and efficient chamfering or beveling of the exposed tube ends of the existing tube wall using conventional milling tools, and to provide ample space between vertically adjacent tubes for welding around the entire circumference of the tube ends, it is highly desirable and generally necessary to remove a portion of the membrane between each pair of exposed tube ends of the existing tube wall. Typically, it is desirable to remove the membrane between adjacent tube ends, which are exposed by removal of the damaged wall section, from the horizontally cut edge of the existing boiler tube wall to a depth which is at least equal to the depth of the frustoconical bevel which is to be milled at the tube end. Typically, membrane material is removed from the horizontally cut edge between two adjacent tubes to a depth of about ⅜ inch, about ¾ inch, or about 1 inch or more from the cut edge.
Heretofore, membrane removal utilizing rotary milling tools on an existing tube of a tube wall has been limited to a cutting sweep equal to the outer radius of a tube. Prior art membrane removal heads are not capable of performing tube cleanup and/or weld overlay removal due to milling head blade configuration or design. The prior art membrane removal heads seize, stutter and/or stop cutting when exposed to weld overlay or tube material. After the membrane material and/or welding overlay material removal, a frustoconical bevel is milled on the tube around the entire circumference thereof. Afterwards, a second tube with a corresponding bevel is matched with the first beveled tube and the tubes are welded together by molten metal or solder placed around the joined tubes, especially at the circumferential groove formed by the adjoined beveled tube ends. It has been found that beveled tubes joined in this prior manner without diameter cleanup or film removal often have weaknesses at the weld area, occasionally due to pitting, which causes air pockets or lack of weld strength. It would be beneficial to form a strong durable weld between joined tubes having beveled or chamfered ends.